An optical instrument for observation includes, in order from the object side, an objective lens system having positive refractive power, an erecting optical system, and an ocular lens system having positive refractive power. When an anti-vibration mechanism is used to maintain the erecting prism at an orientation in space that is stabilized so as to prevent image degradation due to vibrations of the optical instrument for observation, the erecting prism is constructed to satisfy specified conditions so that ghost light is not generated and so that the optical instrument for observation may be compact.
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7. An optical instrument for observation that comprises, in order from the object side:
an objective lens system of positive refractive power that is formed of, in order from the object side, a positive first lens group and a negative second lens group; an erecting prism; and an ocular lens of positive refractive power; wherein
said erecting prism includes, in order from the object side, a first prism and a second prism, and the following Conditions (2) through (4) are satisfied
where
θ is the apex angle of the first prism as measured between the first incident surface of light entering the prism and the next surface the light is incident upon; np1 is the index of refraction of the first prism; fo is the focal length of the objective lens system, and fo1 is the focal length of the first lens group of the objective lens system.
1. An optical instrument for observation comprising, in order from the object side:
an objective lens system having positive refractive power; an erecting optical system; and an ocular lens system having positive refractive power; wherein
an anti-vibration mechanism is used to maintain the erecting optical system at an orientation in space that is stabilized so as to prevent image degradation due to vibrations of the optical instrument for observation; and the erecting optical system is an erecting prism that is constructed so as to satisfy the following Condition (1):
where
L is the distance from the lens element surface nearest the object side of the objective lens system to the erecting prism, fo is the focal length of the objective lens system, and ω is the maximum angle of rotation of the erecting prism relative to the optical axis of the optical instrument for observation in order to maintain the spatial orientation of the erecting prism substantially constant in space when correcting for sudden changes in orientation of the optical axis.
2. The optical instrument for observation according to
where
θ is the apex angle of the first prism as measured between the first incident surface of light entering the prism and the next surface the light is incident upon; and np1 is the index of refraction of the first prism.
3. The optical instrument for observation according to
where
fo is the focal length of the objective lens system, and fo1 is the focal length of the first lens group of the objective lens system.
4. The optical instrument for observation according to
where
fo is the focal length of the objective lens system, and fo1 is the focal length of the first lens group of the objective lens system.
5. The optical instrument for observation according to
6. The optical instrument for observation according to
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Optical instruments such as binoculars and telescopes which include an objective lens and an ocular lens have been known. When both the objective lens and the ocular lens are constructed so as to have positive refractive power, an inverted image is formed. In astronomical telescopes this is acceptable. However, in binoculars and terrestrial telescopes, an erecting optical system is placed between the objective lens and the ocular lens in order that the observer can observe an erect image. For example, the erecting optical system often includes an erecting prism, as used in so-called prism binoculars. More particularly, it is common to use a so-called Schmidt erecting prism in the erecting optical system when making compact binoculars.
FIGS. 7(A) and 7(B) show an optical system of a prior art optical instrument for observation that uses a Schmidt prism to form an erect image. This optical system is provided with a positive objective lens 110, an erecting optical system 130 and a positive ocular lens 120. Moreover, the position "111" on the optical axis Z1 represents the axial position of an image that is formed by the objective lens 110, and "E. P." represents the pupil position for observation (eye point).
As shown in FIGS. 7(A) and 7(B), the erecting optical system 130, termed a Schmidt prism, is actually formed of two prisms, namely, a first prism 131 and a second prism 132 which are almost touching but are separated by a small air gap. The first prism 131 has three reflecting, planar surfaces that are active, namely, surfaces 131A, 131B, and 131C. The second prism 132 is a roof prism and has a roof surface 132C formed of two reflection surfaces that are perpendicular to each other. The second prism 132 has three reflecting, planar surfaces that are active, namely, surfaces 132A, 132B and the roof surface 132C.
In an optical instrument for observation having such an optical system, light emergent from the positive objective lens 110 is first incident upon the surface 131A of the erecting prism 130, at which point it is transmitted. Then the light undergoes total internal reflection at the surface 131B and is directed to the surface 131C, where it again undergoes total internal reflection. When the light is again incident onto surface 131B, its incident angle is less than that required for total internal reflection, and so the light is transmitted. Thus, the light is transmitted through surface 131B and is incident onto the surface 132A of the second prism 132.
The light incident upon the second prism 132 undergoes total internal reflection at the surface 132B, is reflected by a mirror at surface 132C, and undergoes total internal reflection at surface 132A, and then is emergent from the surface 132B and is transmitted to the ocular lens 120. The inverted image that would normally be formed by the positive objective lens 110, by the action of the erecting prism 130, is converted to an erect image. This erect image is then observed by the ocular lens 120.
When a Schmidt erecting prism 130 is used to erect an image, there is a problem in that ghost light, as will be explained below, may be generated. Namely, as shown in FIGS. 7(B) and 8, light 140 that is incident upon the first reflection surface (i.e., the surface 131B of first prism 131) at an angle θ1 (
In optical instruments for observation, such as monoculars and binoculars, if a vibration occurs so as to deviate the optical axis of an optical instrument for observation from the viewing direction, an angular deflection of the light rays occurs. Thus, the quality of an observed image may be greatly degraded, especially in the case where the image is observed with a high magnification. Various anti-vibration mechanisms have been proposed for optical instruments in order to prevent the angular deflection of light rays from an observed object due to a vibration. For example, a mechanism has been disclosed which maintains the spatial orientation of an erecting optical system, such as the so-called Schmidt prism 130, substantially constant despite a vibration or sudden change in the direction of the optical axis of the optical instrument.
However, ghost light is particularly troublesome in an optical system that has been provided with such an anti-vibration or image-stabilizing mechanism. In such an optical system, the spatial orientation of an erecting optical system, such as the Schmidt prism 130, is maintained constant during a vibration while the objective lens 110 and the ocular lens 120 are rotated by the vibration relative to the Schmidt prism 130. When the optical system rotates as a result of a vibration or sudden change in the direction of the optical axis of the optical instrument, the objective lens 110 receives light that originally was outside the effective diameter of the objective lens 110. Some of this light may not undergo total internal reflection in the Schmidt prism, such as at surface 132A, and will produce ghost light that will degrade the quality of the observed image.
The present invention relates to an optical instrument for observation which uses an inverting optical system (erecting system) for forming an erect image, and more particularly, relates to an optical instrument for observation such as binoculars or a telescope, etc., that is provided with an anti-vibration mechanism. In particular, the present invention relates to an optical instrument for observation which can prevent the occurrence of ghost light.
The present invention will become more fully understood from the detailed description given below and the accompanying drawings, which are given by way of illustration only and thus are not limitative of the present invention, wherein:
FIGS. 6(A) and 6(B) are block diagrams showing a Schmidt erecting prism;
FIGS. 7(A) and 7(B) are schematic diagrams showing the construction of an optical system of a conventional optical instrument for observation using a Schmidt erecting prism; and
The present invention is an optical instrument for observation that includes, in order from the object side, an objective lens system having a positive overall refractive power, an erecting optical system, and an ocular lens system having a positive overall refractive power, and in which an anti-vibration mechanism maintains the spatial orientation of the erecting optical system substantially constant in space despite the occurrence of vibrations which change the direction of the optical axis Z1 of the optical instrument. According to a first feature of the invention, the following Condition (1) is satisfied:
where
L is the distance from the lens element surface nearest the object side of the objective lens system to the erecting prism,
fo is the focal length of the objective lens system, and
ω is the maximum angle of rotation of the erecting prism relative to the optical axis Z1 of the optical instrument in order to maintain the spatial orientation of the erecting prism substantially constant in space when correcting for sudden changes in orientation of the optical axis Z1.
In the present invention, an object image that would ordinarily be formed inverted by an objective lens system having positive overall refractive power is instead converted to an erect image by the operation of the erecting optical system. The erect image is then observed by the ocular lens system. By use of an anti-vibration mechanism, the spatial orientation of the erecting optical system is maintained at substantially an initial orientation in space during a vibration that causes the optical axis of the optical instrument to change its orientation.
During a vibration, an erecting optical system is rotated relative to an optical axis that connects the objective lens system and the ocular lens system by an anti-vibration mechanism.
By satisfying the above Condition (1), the occurrence of ghost light is diminished as compared with the case of not satisfying Condition (1). Moreover, the greater the distance L, the less likely ghost light will degrade the image. However, this causes the overall length of the optical instrument to increase. Thus, by satisfying Condition (1), a proper balance is maintained between compactness of the optical instrument and the occurrence of ghost light.
According to a second feature of the present invention, it is desirable that the optical erecting prism be a so-called Schmidt prism, which actually is formed of two prisms, a first prism on which the light is initially incident, and a second prism which receives output light from the first prism, and that the following Conditions (2) and (3) are satisfied:
where
θ is the apex angle of the first prism as measured between the first incident surface of light entering the prism and the next surface the light is incident upon; and
np1 is the index of refraction of the first prism.
By satisfying Conditions (2) and (3), ghost light will be prevented, in that all the light that is incident onto the first reflection surface will be totally internally reflected at the first reflection surface.
According to a third feature of the invention, it is desirable that the objective lens system be constructed of, in order from the object side, a first lens group having positive refractive power and a second lens group having negative refractive power, and that the objective lens system be constructed so as to satisfy the following Condition (4):
where
fo is the focal length of the objective lens system, and
fo1 is the focal length of the first lens group of the objective lens system.
Condition (4) ensures a proper power distribution among the first lens group of the objective lens system as compared with the entire objective lens system. If the lower limit of Condition (4) is not satisfied, correction of spherical aberration of the first lens group will become difficult. If the upper limit of Condition (4) is not satisfied, the overall length of the optical instrument for observation will be such that compactness will be lost.
Satisfying Condition (4) enables the spherical aberration to be well-corrected while ensuring that the overall length of the optical instrument is not excessive. Furthermore, it is desirable that the second lens group of the objective lens system be movable along the optical axis for focus adjustment.
The invention will first be described in general terms with reference to the drawings.
Such an anti-vibration mechanism maintains the erecting prism 30 at an initial orientation in space despite sudden small changes in orientation (herein termed a vibration) which results from instability in directing the optical instrument at a viewed object. In
The objective lens 10 may be constructed as illustrated in FIG. 2. In order from the object side, the objective lens 10 of
For example, Schmidt erecting prisms are shown in FIGS. 6(A) and 6(B). The combination of prisms shown in FIG. 6(A) is an example of combining a roof prism 61 with a wedge-type prism 51. The roof surface 62 here is provided roof prism 61. The Schmidt erecting prism 50B shown in FIG. 6(B) is an example of combining an isosceles triangle prism 52 and a wedge-type prism 71 that includes a roof surface 72 in lieu of, for example, the wedge surface 51C shown in FIG. 5. These Schmidt erecting prisms 50A, 50B function to invert an image whether the incident light travels in the direction indicated by the arrow ZL (entering from the left side) or whether the incident light travels in the direction indicated by the arrow ZR (entering from the right side).
In
Although not illustrated in
The operation of the optical instrument for observation according to the present invention will now be described, with reference to
Light incident upon the second prism 32 is transmitted at its first incidence onto surface 32A, and is then reflected at the surfaces 32B, 32C and 32A so that it is incident onto surface 32B, which transmits it to the ocular lens 20. What would normally be an inverted image at position 11 that is formed by the objective lens 10 of an object is converted by the roof surfaces 32C of the Schmidt erecting prism 30 into an erect image. The lens 20 creates a virtual magnified image of the erect image at 11 (which also is erect) and which may be viewed by an observer placing his eyes at the Eye Point E. P.
In this optical instrument for observation, the spatial orientation of the erecting prism 30 is stabilized by an anti-vibration mechanism, so as to prevent image degradation which occurs if the optical axis of the observing optical system itself is subject to angular motions, which causes light rays passing through the optical instrument for observation to be deflected. As shown in
Referring to
Referring again to
where
D', L and ω are as defined above.
If the upper limit of Condition (5) is exceeded ghost light becomes a problem. On the other hand, if the lower limit of Condition (5) is not satisfied, although ghost light is not generated, the objective diameter becomes unnecessarily large and the observing optical system will no longer be compact.
Conditions (2) and (3) ensure that the light that is first incident onto the surface 31B will be totally internally reflected.
Referring to
where
β is the angle of incidence of the light ray onto the surface 31B, as measured from the surface normal, and
n is the refractive index of first prism 31.
Condition (A) can be expressed as the following Condition (B), using the facts that β=θ-α2 and that sin α1=n sin α2:
From Condition (B), it is known that total internal reflection more easily occurs when the values of θ or n are increased. When total internal reflection occurs at the surface 131B, no light that is first incident the surface 131B is transmitted, as shown in FIG. 7(A). For example, when θ equals 47°C and np1 equals 1.648, the light 140 that is incident upon the reflection surface 131B of the first prism 131 at the critical angle (i.e., the very limit for total internal reflection) is easily outside the effective diameter of the objective lens 110. Therefore no ghost light occurs.
On the other hand, for example, when the angle θ=45°C and the refractive index np1=1.569, the light 140 incident at the critical angle easily comes within the effective diameter of the objective lens 110 and is incident upon the erecting prism 130; thus ghost light occurs, as shown in FIG. 7(B).
In the optical instrument for observation according to the present invention, light is diverged by the second lens group G2 (
Next, two embodiments of an optical instrument for observation will be set forth in detail.
As mentioned above,
Table 1 below lists the surface number #, in order from the object side, the radius of curvature R (in mm) of each surface, the on-axis spacing D (in mm) between surfaces, as well as the index of refraction Nd (at the d-line, i.e., λ=587.6 nm) of each optical element of the optical instrument for observation of Embodiment 1. In the bottom portion of the table are listed various values that relate to Conditions (1)-(5) above.
TABLE 1 | ||||
# | R | D | Nd | |
1 | 75.78 | 7 | 1.516797 | |
2 | -48.65 | 2 | 1.581437 | |
3 | -153.19 | 21.87 | ||
4 | -141.40 | 2 | 1.516797 | |
5 | 325.05 | 29.05 | ||
6 | ∞ | 37.18 | 1.620037 | |
(npl) | ||||
7 | ∞ | 0.80 | ||
8 | ∞ | 58.58 | 1.568830 | |
9 | ∞ | 54.37 | ||
10 | ∞ | 4.89 | ||
11 | -55.65 | 1.60 | 1.784713 | |
12 | 32.76 | 3 | ||
13 | ∞ | 5.50 | 1.620407 | |
14 | -26.14 | 0.40 | ||
15 | 43.42 | 5.70 | 1.696800 | |
16 | -43.42 | 0.40 | ||
17 | 24.76 | 8.50 | 1.696800 | |
18 | -24.76 | 1.60 | 1.784713 | |
19 | ∞ | |||
In this embodiment, the angle θ equals 47°C and the refractive index np1 of the first prism 31 equals 1.620037. As is apparent from comparing these values to the Conditions (1)-(5), this embodiment satisfies each of the Conditions (1)-(5).
As mentioned above,
Table 2 below lists the surface number #, in order from the object side, the radius of curvature R (in mm) of each surface, the on-axis spacing D (in mm) between surfaces, as well as the index of refraction Nd (at the d-line, i.e., λ=587.6 nm) of each optical element of the optical instrument for observation of Embodiment 2. In the bottom portion of the table are listed various values that relate to Conditions (1)-(5) above.
TABLE 2 | ||||
# | R | D | Nd | |
1 | 86.12 | 7 | 1.516797 | |
2 | -56.32 | 2 | 1.581437 | |
3 | -224.13 | 28.25 | ||
4 | -281.80 | 2 | 1.516797 | |
5 | 486.51 | 28.49 | ||
6 | ∞ | 37.18 | 1.620037 | |
(npl) | ||||
7 | ∞ | 0.80 | ||
8 | ∞ | 58.58 | 1.568830 | |
9 | ∞ | 47.37 | ||
10 | ∞ | 4.89 | ||
11 | -55.65 | 1.60 | 1.784713 | |
12 | 32.76 | 3 | ||
13 | ∞ | 5.50 | 1.620407 | |
14 | -26.14 | 0.40 | ||
15 | 43.42 | 5.70 | 1.696800 | |
16 | -43.42 | 0.40 | ||
17 | 24.76 | 8.50 | 1.696800 | |
18 | -24.76 | 1.60 | 1.784713 | |
19 | ∞ | |||
In this embodiment, the angle θ equals 47°C and the refractive index np1 of the first prism 31 equals 1.620037. As is apparent from comparing these values to the Conditions (1)-(5), this embodiment also satisfies each of the Conditions (1)-(5) so as to prevent ghost images and provide a compact optical instrument for observation.
The invention being thus described, it will be obvious that the same may be varied in many ways. For example, values of the radii of curvature R, the on-axis surface spacings D, and the refractive index Nd of the lens components are not limited to the values shown in the above numerical embodiments, as other values can be used. Furthermore, the number of lens components and lens elements in the objective lens 10 and the ocular lens 20 and their power distribution are also not limited to those shown in the above embodiments, as other constructions can be used. Such variations are not to be regarded as a departure from the spirit and scope of the invention. Rather, the scope of the invention shall be defined as set forth in the following claims and their legal equivalents. All such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.
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